Grain varies every pixel at random. Drawn replaces that with real marks — blades in the grass, slabs in the pavement, ripples in the sand. Ink adds a dark line wherever one material meets another.
How small the marks are. The picture underneath keeps its own size: this is the shading on top of it, and it is free.
A bridge deck wears the same tarmac and pavement as the road that runs onto it, and walls get the ink line around their windows and doors.